Cmos image sensor

ABSTRACT

Disclosed is a CMOS image sensor, which can minimize a reflectance of light at an interface between a photodiode and an insulating film, thereby enhancing image sensitivity. Such a CMOS image sensor includes a substrate provided with a photodiode consisting of Si, an insulating film consisting of SiO2 and formed on the substrate, a semi-reflection film interposed between the substrate and the insulating film, and metal interconnections, color filters and micro lenses constituting individual unit pixels. The semi-reflection film has a refraction index value between those of the Si photodiode and the SiO2 insulating film.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to a CMOS image sensor, and moreparticularly to a CMOS image sensor, which can minimize a reflectance oflight at an interface between a photodiode and an insulating film,thereby enhancing image sensitivity thereof.

2. Description of the Prior Art

A CMOS image sensor includes a plurality of pixels, each of whichconsists of several transistors for various purposes and one photodiode.In such a CMOS image sensor, first of all, light transmits thephotodiode made of Si while exciting holes formed in the photodiode togenerate electrical signals and then the electrical signals areconverted into an image.

It is ideal that 100% of light reaches the photodiode, but an area ratiooccupied by the photodiode per pixel is reduced to 30% or less as apixel size becomes smaller and smaller due to the requirement for highpicture quality. That is, only 30% or less of actual incident light canenter into the photodiode. For this reason, there is employed a methodin which a micro lens is formed above each pixel to focus 90% of lighton the photodiode.

Meanwhile, since multilayer metal interconnections are formed above thephotodiode and the transistors, light incident through the micro lenspasses multilayer insulating films and then arrives at the photodiode.In this course, light does not transmit the insulating films by 100%,but is reflected to a certain extent. The larger a difference betweenrefraction indices of materials is, the more light is reflected. Ingeneral, places where reflection occurs most frequently are an interfacebetween a photodiode, a kind of doped silicon, and a SiO2 basedinsulating film. A reflectance at such an interface amounts to about 15%as shown below in Table 1.

Table 1 shows a reflectance at the interface between the photodiode madeof Si and the SiO2 based insulating film used above the photodiode.Here, it can be seen that the photodiode has a refraction index (n) of3.4 and the insulating film used above the photodiode has a refractionindex (n) of 1.5.

TABLE 1 Material n Reflectance Si 3.4 SiO2 1.5 0.150354019 Totalreflectance 15.04

An alternative proposal to reduce the reflectance to 15% or less employsa structure in which a semi-reflection film of SiN having a refractionindex of 2.1 is coated between the photodiode and the insulating film,which results in a reflectance of about 8% However, even if such astructure is employed, there is still a problem in that the sum ofreflectance values occurring at an interface between the micro lens andthe insulating film or between every two layers of the multilayerinsulating films brings about a loss of at least 10%.

SUMMARY OF THE INVENTION

Accordingly, the present invention has been made to solve theabove-mentioned problems occurring in the prior art, and an object ofthe present invention is to provide a CMOS image sensor, which enablelight incident through a micro lens to reach a photodiode consisting ofSi as much as possible by interposing a semi-reflection film having arefraction index value between those of the Si photodiode and a firstinsulating film consisting of SiO2 at an interface between the Siphotodiode and the SiO2 first insulating film, that is, a place wherethe greatest loss of light occurs, to minimize a reflectance at theinterface.

In order to accomplish this object, there is provided a CMOS imagesensor comprising: a substrate being provided with a photodiodeconsisting of Si; an insulating film consisting of SiO2 and being formedon the substrate; a semi-reflection film being interposed between thesubstrate and the insulating film and having a refraction index valuebetween those of the Si photodiode and the SiO2 insulating film; andmetal interconnections, color filters and micro lenses constitutingindividual unit pixels.

Any one single film selected from the group consisting of diamond, SiCand SiNx may be used as the semi-reflection film. Here, when a diamondsingle film is used as the semi-reflection film, it has a refractionindex value of 2.5 to 3.4 and a thickness of 100 to 5000 Å. Similarly, aSiC single film to be used as the semi-reflection film has a refractionindex value of 2.5 to 3.4. Also, a SiNx single film to be used as thesemi-reflection film has a refraction index value of 1.9 to 2.4 and athickness of 100 to 5000 Å.

The semi-reflection film may be formed in a double film structure ofSiNx/SiOxN1−x (0<x<1). Here, the SiOxN1−x film has a refraction indexvalue of 1.5 to 2.1 and a thickness of 100 to 5000 Å.

The semi-reflection film may be formed in any one structure of SiC/SiNxand SiCx/SiNxC1−x/SiOxN1−x. Here, the SiNxC1−x/SiOxN1−x film has arefraction index value continuously varying between 1.5 and 3.4 and athickness of 100 to 5000 Å.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the presentinvention will be more apparent from the following detailed descriptiontaken in conjunction with the accompanying drawings, in which:

FIG. 1 is a sectional view for explaining process-by-process structuresof a CMOS image sensor in accordance with a preferred embodiment of thepresent invention; and

FIG. 2 is a sectional view showing a photodiode/semi-reflectionfilm/insulating film structure in the CMOS image sensor according topresent invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, preferred embodiments of the present invention will bedescribed with reference to the accompanying drawings. In the followingdescription and drawings, the same reference numerals are used todesignate the same or similar components, and so repetition of thedescription on the same or similar components will be omitted.

FIG. 1 illustrates a sectional view for explaining process-by-processstructures of a CMOS image sensor in accordance with a preferredembodiment of the present invention, and FIG. 2 illustrates a sectionalview showing a photodiode/semi-reflection film/insulating film structurein the CMOS image sensor according to present invention.

As shown in FIG. 1, the CMOS image sensor according to the presentinvention includes a substrate 10 provided with photodiodes 12consisting of Si and device separating films 11, a first insulating film14 formed on the substrate 10, a semi-reflection film 13 interposedbetween the photodiode 12 and the first insulating film 14, first metalinterconnections M1 formed on the first insulating film 14, a secondinsulating film 15 formed on the first insulating film including thefirst metal interconnections M1, second metal interconnections M2 formedon the second insulating film 15, a third insulating film 16 formed onthe second insulating film 15 including the second metalinterconnections M2, third metal interconnections M3 formed on the thirdinsulating film 16, a fourth insulating film 17 formed on the thirdinsulating film 16 including the third metal interconnections M3, fourthmetal interconnections M4 formed on the fourth insulating film 17, afifth insulating film 18 formed on the fourth insulating film 17including the fourth metal interconnections M4, RGB color filters 19formed on the fifth insulating film 18, a planarizing film 20 coveringthe respective color filters 19, and micro lenses 21 formed on theplanarizing film 20.

The semi-reflection film 13 has a refraction index value between thoseof the Si photodiode 12 and the SiO2 first insulating film 14. Since theSi photodiode 12 and the SiO2 first insulating film 14 have refractionindex values of 3.4 and 1.5, respectively, the refraction index value ofthe semi-reflection film 13 according to the present inventioncorresponds to a value of 1.5 to 3.4.

As material of the semi-reflection film 13, (1) any one single film ofdiamond, Sic and SiNx, (2) any one double film of SiNx/SiOxN1−x (0<x<1),or (3) any one film of SiC/SiNx and SiCx/SiNxC1−x/SiOxN1−x (shown inFIG. 2) may be used.

When the single film structure is employed as the semi-reflection film13 (corresponding to (1)), the semi-reflection film 13 has (1) arefraction index value of 2.5 to 3.4 and a thickness of 100 to 5000 Å ina case of using a diamond single film, (b) a refraction index value of2.5 to 3.4 in a case of using a SiC single film, or (3) a refractionindex value of 1.9 to 2.4 and a thickness of 100 to 5000 Å in a case ofusing a SiNx single film.

When the double film structure is employed as the semi-reflection film13 (corresponding to (2)), the SiOxN1−x film has a refraction indexvalue of 1.9 to 2.4 and a thickness of 100 to 5000 Å.

When the any one structure of a single composition SiC/SiNx film and aSiCx/SiNxC1−x/SiOxN1−x film is employed as the semi-reflection film 13(corresponding to (3)), in particular, the SiNxC1−x/SiOxN1−x film has arefraction index value continuously varying between 1.5 and 3.4 and athickness of 100 to 5000 Å.

Hereinafter, a description will be given for a method for forming a CMOSimage sensor in accordance with a preferred embodiment of the presentinvention.

First of all, device separating films 11 for electrical insulationbetween devices are formed on a semiconductor substrate 10. At thistime, the device separating films 11 may be formed using a LocalOxidation Of Silicon (LOCOS) method and may have a trench structure.

Thereafter, a gate electrode (not shown) is formed on the substrate 10including the device separating films 11 and then an ion implantationprocess progresses to form photodiodes 12 consisting of Si and ionimplantation for forming sources/drains of transistors and sensing nodesis also performed.

Next, a semi-reflection film 13 and a first insulating film 14 aresuccessively deposited on the whole structure including the deviceseparating films 11 and the photodiodes 12.

With respect to this, (1) when a diamond single film is used as thesemi-reflection film 13, the diamond single film is formed by a ChemicalVapor Deposition (CVD) method using plasma. When a SiC single film isused as the semi-reflection film 13, the SiC single film is formed usinga CVD or PlaSma Enhanced Chemical Vapor Deposition (PECVD) method. Whena SiN single film is used as the semi-reflection film 13, the SiNxsingle film has a refraction index value of 1.9 to 2.4 and preferably arefraction index value of 2.1 in a case of x=1 as shown below in Table2.

TABLE 2 Material N Reflectance Si 3.4 SiN 2.1 0.055867769 SiO2 1.50.027777778 Total reflectance 8.36

Table 2 shows reflectance values when the photodiode/semi-reflectionfilm/first insulating film has a Si/SiN/SiO2 structure (where, ndesignates a refraction index value).

(2) When a SiNx/SiOxN1−x double film is used as the semi-reflection film13, the SiOxN1−x film is formed using a CVD or PECVD method while aratio of a nitrogen source (NH3) and an oxygen source (TEOS or O2) iscontinuously changed with the passage of time. Here, a singlecomposition film or a film having a composition continuously varyingwithin a range of 0<x<1 can be used as the SiOxN1−x film.

(3) When a single composition SiC/SiNx film, a SiC/SiNx film having avariable composition or a SiCx/SiNxC1−x/SiOxN1−x film is used as thesemi-reflection film 13, in particular, in a case of using a singlecomposition SiC/SiN (x=1) film, SiC has a refraction index value of 2.4and SiN has a refraction index value of 2.1 as shown below in Table 3.

TABLE 3 Material N Reflectance Si 3.4 SiC 2.4 0.029726516 SiN 2.10.004444444 SiO2 1.5 0.027777778 Total Reflectance 3.22

Table 3 shows reflectance values when the photodiode/semi-reflectionfilm/first insulating film has a Si/SiC/SiN/SiO2 structure (where, ndesignates a refraction index value).

In case of using the SiCx/SiNxC1−x/SiOxN1−x film as the semi-reflectionfilm (shown in FIG. 2), the SiCx/SiNxC1−x/SiOxN1−x film is formed usinga CVD or PECVD method while quantities of reaction gases are so adjustedas to continuously change the composition as SiC SiN SiOx.

Next, first, second, third and fourth metal interconnections M1, M2, M3,M4 constituting unit pixels are formed on the first insulating film 14.At this time, the first, second, third and fourth metal interconnectionsM1, M2, M3, M4 are disposed such that they do not interrupt lightentering into the photodiodes 12. Also, second, third, fourth and fifthinsulating films 15, 16, 17, 18 are formed between every two layers ofthe metal interconnections M1, M2, M3, M4 constituting individual unitpixels.

Next, red, blue and green color filters 19 are formed on the fifthinsulating film 18, respectively and a planarizing film 20 is so formedas to cover the color filters 19. Finally, a micro lens 21 is fabricatedon the planarizing film 20.

As described above, according to a CMOS image sensor of the presentinvention, a semi-reflection film having a refraction index valuebetween those of a photodiode consisting of Si and a first insulatingfilm consisting of SiO2 is interposed at an interface between the Siphotodiode and the SiO2 first insulating film, that is, a place wherethe greatest loss of light occurs, to minimize a reflectance at theinterface, so that the image sensor enabling light incident through amicro lens to reach the Si photodiode as much as possible.

Accordingly, the present invention can be applied to a CMOS image sensorof a million-pixel-grade or a higher pixel-grade for realizing images ofhigh picture quality.

Although preferred embodiments of the present invention have beendescribed for illustrative purposes, those skilled in the art willappreciate that various modifications, additions and substitutions arepossible, without departing from the scope and spirit of the inventionas disclosed in the accompanying claims.

1-9. (canceled)
 10. An image sensor, comprising: a substrate including aphotodiode; an anti-reflection film disposed over the photodiode to forma first interface between the anti-reflection film and the photodiode;and an insulating film disposed on the anti-reflection film to form asecond interface between the anti-reflection film and the insulatingfilm; wherein a refraction index of the anti-reflection film has a firstvalue at the first interface and a second value at the second interface.11. The image sensor of claim 10, wherein the refraction index of theanti-reflection film varies in a stepped gradient function from thefirst value to the second value.
 12. The image sensor of claim 10,wherein the refraction index of the anti-reflection film variescontinuously from the first value to the second value.
 13. The imagesensor of claim 10, wherein the refraction index of the anti-reflectionfilm increases from the first value to the second value.
 14. The imagesensor of claim 10, wherein the first value is closer to a refractionindex of the photodiode than the second value is to the refraction indexof the photodiode.
 15. The image sensor of claim 10, wherein the secondvalue is closer to a refraction index of the insulating film than thefirst value is to the refraction index of the insulating film.
 16. Theimage sensor of claim 10, wherein the anti-reflection film comprises aplurality of layers that each have a different refraction index.
 17. Theimage sensor of claim 10, wherein the first value is about 3.4 and thesecond value is about 2.5.
 18. The image sensor of claim 10, wherein thefirst value is about 3.4 and the second value is about 1.5.
 19. Theimage sensor of claim 10, wherein the first value is about 2.1 and thesecond value is about 1.5.
 20. The image sensor of claim 10, wherein theanti-reflection film has a thickness of about 100 Å to about 5000 Å. 21.A method of fabricating an image sensor, the method comprising: forminga substrate having a plurality of photodiodes; forming ananti-reflection film on the photodiodes; and forming an insulating filmover the anti-reflection film; wherein said forming an anti-reflectionfilm includes varying a refraction index of the anti-reflection filmfrom a first value at a first interface with the plurality ofphotodiodes to a second value at a second interface with the insulatingfilm.
 22. The method of claim 21, wherein said varying a refractionindex of the anti-reflection film comprises: using a nitrogen source andan oxygen source to form the anti-reflection film; and changing a ratioof the nitrogen source and the oxygen source during formation of theanti-reflection film.
 23. The method of claim 21, wherein said varying arefraction index of the anti-reflection film comprises: varying therefraction index of the anti-reflection film in a stepped gradientmanner from the first value to the second value.
 24. The method of claim21, wherein said varying a refraction index of the anti-reflection filmcomprises: varying the refraction index of the anti-reflection filmcontinuously from the first value to the second value.
 25. The method ofclaim 21, wherein said varying a refraction index of the anti-reflectionfilm comprises: increasing the refraction index of the anti-reflectionfilm from the first value to the second value.
 26. The method of claim21, wherein said forming an anti-reflection film comprises forming theanti-reflection film such that the first value and the second value arebetween a refraction index of the plurality of photodiodes and arefraction index of the insulating film.
 27. The method of claim 21,wherein said forming an anti-reflection film comprises forming theanti-reflection film such that the first value is closer to a refractionindex of the plurality of photodiodes than the second value is to therefraction index of the plurality of photodiodes.
 28. The method ofclaim 21, wherein said forming an anti-reflection film comprises formingthe anti-reflection film such that the second value is closer to arefraction index of the insulating film than the first value is to therefraction index of the insulating film.
 29. The method of claim 21,wherein said forming an anti-reflection film comprises changing sourcematerials during a chemical vapor deposition process used to: form theanti-reflection film; and vary the refraction index from the first valueto the second value.
 30. The method of claim 21, wherein said forming ananti-reflection film comprises changing source materials during a plasmaenhanced chemical vapor deposition process used to: form theanti-reflection film; and vary the refraction index from the first valueto the second value.